The kinetics of pluripotential stem cells (assayed as CFU-S in the mouse) will be studied after partial body irradiation following marrow transfusion, using the difference between male and female karyotypes for identification of donor cells. Preliminary data indicate that for colonization of donor marrow in the recipient to take place, the pluripotential stem cells must be triggered into division and sites normally occupied by the stem cells to be emptied for nidation of newly arrived cells. The proliferative differences between stem cells as related to their self-replicative capacity will also be studied, as will the chemical induction of tolerance to specific antigens. The potential clinical significance of the investigations lies in the possible reduction in the amount of marrow needed for transplantation and possible prevention of graft vs. host reaction.